1. Field of the Invention
The present invention relates to a spark plug for use in ignition of an internal combustion engine.
2. Description of the Related Art
A spark plug is conventionally used for ignition of an automobile engine. A spark plug generally includes: an insulator holding a center electrode on the leading end side of an axial hole and a connecting terminal on the trailing end side; a metal shell holding the insulator while enclosing the trunk portion thereof; and an earth electrode having one end thereof welded to the leading end of the metal shell and its other end opposed to the leading end of the center electrode to form a spark discharge gap.
The center electrode and the connecting terminal are electrically connected in the axial hole of the insulator through a conductive sealing member (or a sealing layer) (as disclosed, for example, in JP-A-2003-22886). Generally, the conductive sealing member is made from a mixture of a metal and glass to impart conductivity by dispersing metal powder into insulating glass. The center electrode and the connecting terminal are fixed in the axial hole by means of the sealing member.
In manufacturing the spark plug, the center electrode and the connecting terminal in the axial hole of the insulator are fixed in the following manner. First, the center electrode is inserted into the axial hole of the insulator from the trailing end side and is retained on a stepped portion in the axial hole, and the axial hole is filled from the trailing end side with the sealing member powder. Next, at a glass sealing step, the insulator is inserted into a heating furnace so that the sealing member is softened, and a connecting terminal is press-fitted from the trailing end side of the axial hole and sintered. Through these steps, the center electrode and the connecting terminal are fixed to seal the axial hole. This sealing step is called a “glass sealing step”.
3. Problems to be Solved by the Invention
In order to realize a higher engine output, the valve employed for intake and exhaust is large-sized in recent years so as to considerably vibrate the engine. Vibration shocks thus applied to the spark plug which is mounted in the engine are also applied to the sealing member through the center electrode. However the sealing structure of a conventional spark plug may have insufficient shock resistance. To solve this problem, the shock resistance can be enhanced, for example, by increasing the content of the metallic component in the sealing member. However, the fluidity of the resulting sealing member is lowered such that it cannot flow sufficiently into the clearance between the inner circumference of the axial hole of the insulator and the outer circumference of the connecting terminal. This in turn causes another problem in that the connecting terminal is inadequately fixed to the insulator.
The present invention has been conceived in order to solve the aforementioned problem, and an object thereof is to provide a spark plug which can enhance the gas-tightness of an axial hole of an insulator and which can ensure adequate fixing of a connecting terminal and a center electrode in the axial hole.